Lake Toba is the largest volcanic lake in the world — 100 kilometres long, 30 across and more than 500 metres deep — with an island roughly the size of Singapore sitting in the middle of it. That island is Samosir, and it carries villages, roads and rice fields of its own. The lake rests at 905 metres in the highlands of North Sumatra, cool and pine-scented where the tropics should be hot. All of it fills the caldera left by an eruption around 74,000 years ago, the largest the planet has seen in 25 million years. When the emptied magma chamber collapsed, the ground caved in, and rain and rivers slowly filled the hole.
The lake is the homeland of the Toba Batak, and their villages line the shore. At Tomok and Ambarita stand stone sarcophagi and the carved seats where Batak kings once judged disputes. To the north the Sipiso-piso waterfall drops about 120 metres off the caldera rim in a single thin ribbon whose Batak name means "like a knife." Mornings are the water's best hour, before the wind gets up. Samosir carries two small lakes of its own, Sidihoni and Aek Natonang, so the island in the lake has lakes inside it. It was not always an island: a canal cut through the isthmus at Pangururan in 1906 separated it from the Sumatran shore, and a bridge has spanned the cut ever since. Toba remains one of Southeast Asia's quieter great landscapes, better known to Indonesians than the wider world.